1. Field of the Invention
The present invention relates to a surface acoustic wave device for use in high frequency (UHF band) and a method for manufacturing the same, and more particularly to a technology relating to improvements of an overall characteristic and a cost reduction of the surface acoustic wave device.
2. Description of the Prior Art
In recent years, the surface acoustic wave device has been widely used in regions of IF band and VHF band such as in an intermediate frequency filter for a television receiver of 50 MHz-60 MHz band. In a high frequency surface acoustic wave device for use in UHF band, a width of electrode fingers of input and output interdigital electrodes (electrode fingers of metal film grating type reflector) is no more than 2 .mu.m and a thickness of the film is less than 0.25 .mu.m. The film is usually made of Al thin film. While a few technical reports which disclose specific numerical data have been known, some of them disclose relatively large values of film thickness. For example, manufacturing methods using wet chemical etching are reported in the Technical Report of the Institute of Electronics and Communication Engineers of Japan, 77 (171), 1977 and Journal of Association of Acoustic of Japan, 33 (10), October 1977, methods using lift-off technique are reported in 1977 Ultrasonic Symposium Proceedings, pages 792-797 and J. Electrochemical Society, 121 (11), pages 1503-1506. In those reports, the thickness of the Al film of the electrode finger is no more than 0.2 .mu.m.
In the prior art surface acoustic wave device for use in the high frequency band, the width of the electrode finger is no more than 2 .mu.m and the film thickness of the narrow electrode finger is no more than 0.2 .mu.m as described above mainly because of a subsidiary acoustic effect encountered when the thickness of the Al film is thicker than the above thickness, for example large dispersion of effective surface acoustic wave velocity of the interdigital electrode and large reflection between and in the input and output electrodes. Those facts caused the designers and the researchers of the surface acoustic wave device to have a feeling of risk of reduction of design margin of characteristics when the film thickness is increased.
At least two secondary adverse effects have been reported which usually occur in the characteristics when the film thickness of the electrode finger is increased. The first effect is the increase of dispersion of effective surface acoustic wave velocity at the electrode fingers of the interdigital electrode. (1974 Ultrasonics Symposium Proceedings, pages 321-328). As an experimental example, the Technical Report of the Institute of Electronics and Communication Engineers of Japan, 75 (252), pages 25-32, 1975 and 1976 Ultrasonics Symposium Proceedings, pages 432-435 show that the dispersion increases as the film thickness increases. The other effect is that the reflection of the grating type reflector of the metal film increases but the reflection of the electrode fingers of the input and output interdigital electrodes also increases, and disturbance of band characteristic and ripple due to triple transit echo (TTE) are likely to occur.
Accordingly, it is considered that the use of the electrode having the film thickness of no more than 0.2 .mu.m has been mandatory in designing the device.
The second reason for limiting the thickness of the Al film to no more than 0.2 .mu.m for the electrode fingers having the width of no more than 2 .mu.m is considered to be due to the difficulty in the manufacture as explained below. In a conventional chemical etching process frequently used in the past, the etching proceeds not only normally to the Al film surface but also laterally of the Al film (as reported in Handbook of Thin Film Technology, pages 7-45, Maissel and Glang, 1970) and a large undercut takes place because the etching solution penetrates into a boundary surface of a photoresist and the Al film. For example, the Technical Report of The Institute of Electronics and Communication Engineers of Japan, 77 (259), pages 9-16, March 1978 reports that when the line width of the electrode was 0.58 .mu.m and 0.7 .mu.m, the line width was reduced by 0.2 .mu.m even when the film thickness was no more than 0.043 .mu.m. In the lift-off process, a thin Al film must be used in order to facilitate the removal of resist and at the same time a temperature of a piezoelectric substrate must be maintained at a low temperature during deposition of the Al film. Under those conditions, in order to enhance the adhesability of the Al film to the substrate, Cr or Ti must be previously vapor-deposited. This requires additional steps and cost. An ion etching process by ion shower is advantageous for fine etching but since it is a physical etching process by ion bombardment a selection between the photoresist and the Al film is low and the Al film thickness is restricted and the surface of the substrate is significantly damaged. (Journal of Association of Electronics and Communication Engineering of Japan, 60 (11), page 1259, November 1977). Accordingly, the film thickness of approximately 0.2 .mu.m has been necessarily considered as a barrier.
In addition to the above design problems and the manufacturing process problems, the following problems are encountered in the high frequency surface acoustic wave device which is used in a higher frequency and has an electrode finger width of no more than 2 .mu.m and a film thickness of less than 0.25 .mu.m. (1) Because of very thin film thickness, defect such as break of the electrode finger due to pinholes is likely to occur and a yield is low. (2) Because of large resistance of the electrode fingers of the input and output interdigital electrodes, a loss is high. (For example, 31st Annual Frequency Control Symposium Proceeding, pages 281-284, reports that the loss of 16-17 dB at the thickness of 0.16 .mu.m increased to 19-20 dB at the thickness of 0.1 .mu.m). (3) When an Al film grating type resonator is used, the number of grating electrodes increases significantly (200-300 electrodes) and a chip area increases accordingly. This leads to the further reduction of yield due to the pinholes. As an example in a VHF band, The Journal of the Institute of Electronics and Communication Engineers of Japan, Vol. 1, J 60-A (9), pages 875-876, September 1977, reports that more than 200 parallel-connected Al electrodes each having a thickness of 0.37 mm must be arranged on a LiNbO.sub.3 substrate in order to attain a reflection efficiency of no less than 0.9 at 160 MHz, and more than 300 parallel-connected Al electrodes each having a thickness of 0.48 .mu.m must be arranged on a quartz substrate to attain the same reflection efficiency. This means that a substrate length in the direction of propagation of the surface acoustic wave of approximately 2 mm for the LiNbO.sub.3 substrate and approximately 3 mm for the quartz substrate is additionally required. If the film thickness is less than 0.25 .mu.m, more number of electrodes are required. If an Al film thinner than the conventional thickness of 0.2 .mu.m is used to form a grating type reflector having a reflection factor of no less than 0.9 at 500 MHz, a length of no less than 1 mm is required. When an electrode of Au 0.2 .mu.m/Cr 0.05 .mu.m was used, a 50-line grating type reflector presented approximately 20 dB of attenuation at 170 MHz, but since the Au electrode has a large mass, it has a very large dispersion of surface acoustic wave velocity and hence it is not appropriate to use when the high frequency input and output interdigital electrodes and wide band elements coexist on a common substrate. (4) In a device in which not only the UHF band input and output electrodes and grating type reflector but also the VHF band input and output electrodes (no higher than 300 MHz) and grating type reflector are to be formed on a common substrate, if an Al film thinner than the conventional thickness of no more than 0.2 .mu.m is used, the number of electrodes of the VHF band grating type reflector increases as seen from the previous example and hence the area of the substrate increases. Contrarily, if the film thickness of the VHF band devices is increased in order to reduce the number of electrodes of the VHF band grating type reflector, the steps of Al film deposition and photoresist application increase. This eventually increases the cost in view of yield, substrate area and the number of steps. (5) It is difficult to maintain bonding with the Au wires in a stable state for a long time period. To resolve the problem, the thickness of only that portion of the Al film to which the Au wire is wire-bonded may be locally increased but this increases the number of steps. By bonding Al wire instead of Au wire by an ultrasonic bonding method, long life bonding may be attained but this method has lower workability than the Au wire bonding method and makes automation and mass production difficult.
As described above, the prior art high frequency band surface acoustic wave devices have many problems and are expensive. This has made the application of the devices to commercial equipments difficult.